The billionaire shareholder activist lashed out at a third eBay board member on Monday for botching the 2009 sale of Skype.

In the latest in a series of scathing letters to shareholders in the San Jose, Calif., company, Icahn said CEO John Donahoe’s “inexcusable incompetence” in handling the Skype sale cost shareholders $4 billion.

Icahn is pressuring the board — including direct attacks on directors Marc Andreessen and Scott Cook — to get them to agree to spin off its lucrative PayPal unit.

Icahn, in his latest letter, said he has “newly uncovered” evidence showing that Donahoe received key information leading up to eBay’s sale of Skype that could have allowed eBay to make a killing off the online voice company — rather than selling it on the cheap to an investment group that included an eBay director.

EBay sold Skype to an investment group that included Andreessen for $1.9 billion. The group turned around and sold it to Microsoft for $8.5 billion just 18 months later, which resulted in another $1.4 billion for eBay.

In lashing out against Donahoe, Icahn said the CEO had received information from a Skype insider that would have allowed eBay to make a sale to Microsoft directly — rather than through Andreessen’s group.

Icahn’s claim is “false and misleading and … utterly discredited by the facts,” eBay said in a statement.

At the same time, eBay on Monday asked shareholders to reject Icahn’s proposal to add two of his employees to eBay’s board, and put forth its own slate of four candidates, including Cook and Donahoe.

Andreessen isn’t up for re-election until 2015.

Icahn said the information that could have allowed Donahoe to lead the sale of Skype to Microsoft came from Mike Volpi, a former Skype director and CEO of Joost, an Internet TV service that was founded by the same people who originated Skype.

According to Icahn, Volpi told Donahoe and Andreessen “that there could be a workaround” of the technology that reportedly caused Microsoft to walk away from buying the online voice company from eBay in 2009.

Icahn, who owns 2 percent of eBay, also received a jolt of shareholder support for his controversial plan for eBay to spin off PayPal, its financial transactions arm.

“In this case, we happen to agree with him,” said fellow hedgie Leon Cooperman, whose Omega Advisors also owns 2 percent of eBay.